The present invention relates to a method of judging the slag foaming in steel manufacture by electric furnace and also to a method of operating the electric furnace utilizing that. More particularly, it relates to an art where a suitability of slag foaming generated during the refining process in an electric furnace is judged by measuring the NOx amount in an exhaust gas and also to an art of conducting the operation by completely inhibiting the contact of melted steel and air by means of adjusting said slag foaming state.
Method of manufacture of steel using electric arc furnace has the following advantages as compared with the conventional steel manufacture method in which blast furnace and converter are combined with an object of a mass production (hereinafter, referred to as xe2x80x9cblast furnace-converter steel manufacture methodxe2x80x9d).
[1] initial cost for investment is small as compared with a blast furnace-converter steel manufacture method;
[2] adjustment of production amount is easy; and
[3] it is able to easily deal with various changes in the main materials.
Therefore, as a result of recognition of those advantages, the cases where steel manufacture method using electric furnace is selected are recently increasing for the manufacture of melted steel.
Incidentally, many of the melted steel obtained by an electric furnace steel manufacture method contains much tramp elements such as Cu, Sn and Cr as compared with a converter steel because the main materials therefor are various kinds of iron scrap and, in addition, its nitrogen content is as high as 70-120 ppm whereby it has been usually used for the manufacture of the so-called low-class steel products such as steel bars and sections. With regard to the fact that there are much amount of tramp elements such as Cu, Sn and Cr among the above reasons, that can be partly dealt with by controlling the stock and supply of the iron scrap. However, the fact of high nitrogen content is a cause of internal crack and surface crack of cast iron pieces during a continuous casting and deterioration in ductility and surface scratch of steel materials during hot rolling. Even when it is made in to a steel product, tensile strength and aging are deteriorated in wire rods and, in the case of cold rolled thin sheet, yield strength increases and deep drawability is deteriorated. Accordingly, when development of the melted steel manufactured by an electric furnace steel manufacture to high-class steel is considered, a reduction of the nitrogen content is a big problem which is unavoidable and inevitable.
The fundamental phenomena per se of denitrification and nitrification reactions of melted steel in an electric furnace are as same as those in a converter and it is believed that the nitrogen content in the melted steel is decided upon its outgoing from the furnace when denitrifying and nitrifying rates are balanced. On the contrary however, as compared with the steel manufacture by a converter, the electric furnace steel manufacture has big differences in terms of equipment and operation as given below.
(1) The furnace is substantially in an open system (note that, particularly near the electrodes, sealing is difficult) and, as compared with a converter, much amount of air is sucked and the nitrogen content in the atmosphere in the furnace is much.
(2) In the arc, the components of the atmospheric gas are atomized and the melted steel is apt to suck the nitrogen in the atmospheric gas at an arc spot (which is a contact point of the arc with the melted steel).
(3) As compared with a converter, generation of carbon monoxide gas is small and, even under reduced pressure, denitrification is small.
In view of the above, a reduction in the nitrogen content in the melted steel by an electric furnace steel manufacture has been conducted by the use of materials containing high amount of carbon such as reduced iron or pig iron or by bubbling a carbon material into the melted state so that the so-called slag foaming is promoted in which the carbon monoxide gas generation from the melted steel surface is increased. In other words, contact of the atmospheric gas with the melted steel surface is interrupted by means of the foaming slag whereby a reduction in amount of sucked nitrogen caused by the above (1) and (2) and a promotion of denitrification by the effect of (3) have been attempted. For example, in the Japanese Laid-Open Patent Publication Sho-53/43003, there is a disclosure on an art in which xe2x80x9cupon dissolution of the materials to be dissolved, an oxidizer such as mill scale dust or the like is added to the melted steel in an amount to make
[Effective Oxygen Concentration (wt %) in Oxidizer]xe2x89xa7[(Carbon Concentration in Scrap (wt %)xe2x88x920.12)xc3x974/3]
whereupon the carbon concentration in a steel bath during the above-mentioned material is melted down is made not more than 0.12 wt % and then carbon material is bubbled into such a melted steel via a carrier gas until the aimed carbon amount is achievedxe2x80x9d.
Further, in the Japanese Laid-Open Patent Publication Hei-03/28312, there is a disclosure on the art that xe2x80x9cinto a melted steel in an electric furnace is bubbled solid carbon, aluminum ash or flux (a slag foaming agent) by means of a carrier gas which is the gas generated from coke oven, blast furnace or converter or a mixture thereofxe2x80x9d. Thus, coke furnace gas, blast furnace gas or/and converter gas is/are used as a means for increasing the carbon concentration in the melted steel after it is melted down. Furthermore, in the Japanese Laid-Open Patent Publication Sho-52/147513, there is a disclosure on the art that xe2x80x9cin an arc furnace for steel manufacture, electrode is made hollow and one or more of the inert gas such as argon and the reductive gas such as hydrogen carbide is/are supplied thereto and, at the same time, the carbon amount in the melted steel when it is melted down is made 0.1% or more while an average temperature rising rate in a reductive stage of the melted steel is made 10xc2x0 C./minute or slowerxe2x80x9d.
However, although all of the above-mentioned prior art literatures are effective for promotion of slag foaming, any of them is unable to control the degree of slag foaming (hereinafter, referred to as xe2x80x9cstatexe2x80x9d) or to confirm the effect resulted thereby. Therefore, even when the above art is adopted, an operator will just conduct an operation to such an extent that the bubbling rate of carbon material is changed by checking the state in the furnace and operator does not substantially conduct an operation confirming the effect of slag foaming for preventing the nitrogen absorption. Accordingly, it is unavoidable that the state of slag foaming differs for each of the charges and it is difficult to steadily prepare the melted steel containing low nitrogen.
In addition, the principle of denitrification is understood as diffusion and discharge of nitrogen into carbon monoxide foams generated in the steel bath in the case of steel manufacture by an electric arc furnace as same as in the case of the converter steel manufacture. Therefore, the more the decarburization rate from the melted state, the more the denitrification. However, even if generation of carbon monoxide gas is increased to promote the denitrification, the outcome will be all right during the carbon monoxide gas is generated but, if the nitrification rate from the above-mentioned arc spot becomes higher than the denitrification one at the final stage of the operation where generation of carbon monoxide gas decreases, the nitrogen content in the melted steel increases together with the progress of the operation. Especially in the case of operation in an electric furnace, it is usual that the temperature of the melted steel does not arrive the aimed outgoing steel temperature even when the carbon concentration in the melted steel is low. Since nitrifying amount at the heating process is big in such a final phase of the operation, the nitrogen amount in the melted steel at the outgoing stage is often in the same degree as that in the conventional operations even if bubbling of carbon material is conducted by various methods. Accordingly, it is difficult to steadily obtain the melted steel containing low amount of nitrogen even if bubbling of carbon material is conducted or a material containing high amount of carbon is used by adopting the above-mentioned prior art literatures. Further, some carbon materials contain much amount of nitrogen and, therefore, there is a case where bubbling of carbon material rather results in an increase of the nitrogen amount in the melted steel whereby selection of due carbon material is necessary as well.
In addition, xe2x80x9ca method of supplying an inert gas or a reductive gas from the hollow of a hollow electrodexe2x80x9d as mentioned in the above Japanese Laid-Open Patent Publication Sho-52/147513 is surely effective in suppressing the nitrification from an arc spot but, on the other hand, electrodes are made hollow and amounts of inert gas and reductive gas significantly increase resulting in a rise of a cost whereby there is a problem that said method is hardly applicable for practical use.
Under such circumstances, an object of the present invention is to offer a method for the operation of electric furnace where the adaptability of the slag foaming state generated in an electric furnace steel manufacture can be judged by measuring the NOx amount in exhaust gas and, at the same time, the foaming state is adjusted to completely interrupt the contact of melted steel with air so that low nitrogen amount is always achieved upon outgoing of the steel or reduction of electric power consumption rate is achieved.